Multiple Input Multiple Output (MIMO) technology plays an import role in increasing a peak rate and system spectral utilization, so such radio access technical standards as Long Term Evolution (LTE) and LTE-Advanced (LTE-A) are constructed on the basis of an MIMO and Orthogonal Frequency Division Multiplexing (OFDM) technology. A performance gain of the MIMO technology is derived from a spatial freedom degree capable of being acquired by a multi-antenna system, and a larger data transmission volume is acquired through a larger spatial freedom degree. Hence, the most important development direction of the MIMO technology lies in the extension of dimensions.
In order to further improve the MIMO technology, a massive MIMO antenna technology has been introduced into a mobile communication system. A full-digital massive MIMO antenna includes 128, 256 or 512 antenna elements, 128, 256 or 512 transceivers each connected to one antenna element, and 128, 256 or 512 digital antenna ports. In order to make fully use of the spatial freedom degree caused by the 128, 256 or 512 digital antenna ports, a UE needs to acquire or know spatial channel information about the 128, 256 or 512 digital antenna ports. The acquisition of the spatial channel information directly depends on the number of Channel State Information-Reference Signals (CSI-RSs) used by the 128, 256 or 512 digital antenna ports. A large amount of CSI-RSs may lead to a remarkably large time-frequency resource overhead.
The full-digital massive MIMO antenna is confronted with such a technical problem as the time-frequency resource overhead caused by the large number of CSI-RSs. Hence, there is an urgent need to provide a method for feeding back the CSI, so as to reduce the number of the CSI-RSs.
Currently, for the full-digital massive MIMO antenna, there are usually two methods for feeding back the CSI on the basis of the CSI-RS. One method is based on the CSI-RSs from an antenna unit, i.e., non-beamformed CSI-RSs, and the other method is based on beamformed CSI-RSs.
However, there are some deficiencies for above two methods. For the method based on the non-beamformed CSI-RSs, the antenna unit for each antenna port provides a relatively small gain, usually 5 to 8 dBi for a directional antenna. In this regard, the CSI-RS received by the UE through each antenna port has relatively small power. For the UE within a coverage range of a cell, there are such defects as low received power or loss of the CSI-RS. Usually, this case is called as inadequate CSI-RS coverage, mainly in terms of the coverage range. Hence, the CSI estimated on the basis of the CSI-RS may be inaccurate, and thereby the system performance may be adversely affected.
For the method based on the beamformed CSI-RSs, N beamformed CSI-RSs with a large gain are configured. Due to the beamforming gain caused by the N beamformed CSI-RSs, there is no inadequate coverage range like the non-beamformed CIS-RS, but there may exist an inadequate coverage angle. This because, on one hand, the N beamformed CSI-RSs are associated with an angular range, and in order to cover the entire angular range, N needs to have a large value, resulting in the remarkably large time-frequency resource overhead. If N has a small value, it is impossible to cover the entire angular range. On the other hand, a distribution range of the UEs is unknown to a base station. In the case that the N beamforming CSI-RSs are configured specially for the UEs within a specific range, it is impossible to provide the beamforming gain to the UEs beyond this range, and thereby the system performance may be adversely affected.